1. Field Of The Invention
This invention relates generally to a method of nondestructive evaluation of physical structures, and more particularly to a method of detecting and evaluating subsurface structural irregularities of objects using thermographic images and associated data.
2. Background Of The Art
Infrared (IR) imaging methods have shown considerable promise as techniques for nondestructive evaluation (NDE) of metals, composites, ceramics, and polymers, offering fast, wide area, noncontact detection of delamination, disbonding, corrosion, and voids. However, optimum results obtained with IR techniques existing in the art have generally required considerable sophistication on the part of the operator, particularly in terms of interpretation and understanding of the resulting IR images.
For example, known methods in the art of infrared NDE involve thermally exciting a target object with a heat source, such as a hot air gun, for a finite period of time, thereby imparting thermal energy to the object. After the target object has been thermally excited, the object is allowed to cool, and thermal energy earlier absorbed by the object then begins to dissipate. As the object cools, a series of successive positionally-fixed IR images of the external surface of the object are acquired by an infrared camera. The images are acquired at fixed time intervals, starting relative to the end or the beginning of the thermal excitation period, and continuing during the period while the object cools, until a pre-determined time period has elapsed.
Typically, the infrared camera communicates with a general purpose computer or other video monitor on which the acquired images are displayed. An operator typically performs NDE analysis of the acquired images by either viewing the video output of the infrared camera in real time, or by having the output data stored to videotape or computer memory, where further processing and analysis may take place. Each image is comprised of a collection of gray-scale (or color) pixels, each pixel corresponding to a specific position on the object's external surface. The amplitude of each pixel, i.e. each distinct shade of gray (or different color) exhibited by the different pixels represents a different thermal energy level associated with the pixel's corresponding position on the object's external surface. Known methods of analysis can be broadly categorized as either visual analysis--where the operator views the image sequence taken during or after heating, watching for changes in the scene which correspond to effects due to subsurface structural irregularities--or temporal analysis--where the behavior of each pixel in the scene is analyzed as a function of time, and changes in slope or amplitude of each pixel over time are correlated to the effects of local subsurface structural irregularities.
The visual method depends upon the ability of an operator to identify anomalous changes in the scene from one image to the next merely by visually inspecting the many thousands of pixels which comprise each and every image. Though widely used as one of the existing alternatives in NDE analysis, the visual method is inherently qualitative and imprecise, since the ability of the operator to identify anomalous changes in the scene depends on subjective factors such as the colors assigned to various temperatures, the quality of the display device, and the experience of the operator. Furthermore, the mere fact that the visual method requires an operator to manually inspect each acquired image makes this method labor intensive and time-consuming.
The temporal method, on the other hand, which can sometimes be more precise than the visual method, is extremely computation intensive and frequently requires that high-resolution, high-speed IR cameras be used in order to acquire sufficient data to determine the accurate shape of the temperature time curve for each pixel of each image and the exact time at which the peak or peak slope of each curve occurs. While more objective and precise than the visual method, the temporal method is not commercially viable due to the high cost of high-resolution, high-speed IR imagers and the computer hardware requirements to perform the necessary complex computations.
Accordingly, there is a need to provide an objective and quantifiable method of NDE analysis which does not require an inordinate amount of complex computations and thus overly expensive hardware.